The present invention relates to strain detection and, more particularly, to the detection of strain distributed along an optical transmission line using an optical interrogation signal.
Various apparatus and systems have been developed for distributed strain or stress detection. In the electrical domain, for example, the effects of strain on a transmission line can be detected by time-domain reflectometry techniques in which an electrical interrogation pulse of known characteristics is transmitted on a transmission line. Any strain-induced faults in the line will alter the characteristic impedance of the line to some extent or the other and reflect part of the interrogation pulse to its source. The distance between the interrogation signal source and the strain point can be determined from the round trip time for the transmission of the interrogation pulse and the reception of the reflected pulse and from the propagation velocity within the line. In an analogous manner, the strain-induced faults or discontinuities in an optical fiber can be determined by launching a defined optical interrogation pulse into a fiber that is subjected to strain and measuring the elapsed time from the launching of the interrogation pulse to the reception of the reflected pulse.
The measurement of the strain on an elongated energy transmitting line has application in composite structures as used in aircraft and spacecraft. Composite materials, such as graphite/epoxy laminates, provide significant increases in strength-to-weigh performance compared to traditional metal structures, principally aluminum alloy, used in airframes. In applications where the structure is subjected to recurring time-varying stress as typically encountered in an aircraft application, the composite tends to fail catastrophically in an unpredictable manner and without advanced warning. Metal structures, in contrast, tend to fail by first developing micro-cracks which propagate with continued stress in a reasonably predictable manner until a total failure occurs. Various techniques, including Magna-flux type detection systems, are available to reliably detect micro-cracks in metal structures prior to their failure. Composite structures, on the other hand, are not well-suited for existing micro-crack detection techniques and, accordingly, a problem is presented in the non-destructive detection of pre-failure indicia.
Discrete strain gauges are typically used to sense strain at a defined location and can be attached to a composite structure. However, discrete location sensing may not provide meaningful information for a large composite structure and the need for wiring between the sensors and a central controller creates a practical upper limit for the number of strain sensors that can be employed. Additionally, a large number of discrete sensors attached to a composite structure can represent a significant cost disadvantage as well compromise the design flexibility of the system.
While the sensing of the structural integrity of a composite structure has principal utility in airframe evaluation, the sensing of the structural integrity also is useful in the military environment in which the structure is subjected to ballistic impact. A sensing system for a composite structure would ideally be able to function as an `intelligent` structure and provide an automatic and reliable assessment of structural integrity immediately after a ballistic impact.
One type of sensing system that has been suggested as suitable for structural integrity sensing of composite structures has been presented by Kingsley, S. and Davies, D. in Electronic Letters OFDR Diagnostics for Fibre and Integrated-Optic Systems, May 9, 1985, Vol. 21, No. 10, Pg. 434-5. The Kingsley system relies on the frequency-altering effects of strain on an optical interrogation pulse in contrast to the time-domain effects described above. The system includes a laser diode that is driven by a time-varying current pulse to produce a frequency varying optical chirp that is launched into an optical fiber. As the frequency varying interrogation pulse propagates along the optical fiber, selected frequency components of the interrogated light are subject to backscatter toward the source with the backscatter at any point in the line being a function of the attendant strain at that point. The amplitude of the return signals is a function of the backscatter at the strain location as well as the cumulative effect of impurities, inclusions, micro-bending, and other attenuation producing factors. The light backscattered from the fiber is optically mixed with a reference signal from the source laser, and beat frequencies are produced with the beat frequency related to the position of the associated strain along the fiber. The Kingsley system represents a device that determines the optical loss along an optical waveguide and functions as a passive, open loop distributed intensity sensing device. Since the returned optical signal includes both an information signal and the equivalent of background noise caused by the cumulative effect of core imperfections, the signal-to-noise ratio of the Kingsley system diminishes with increased sensing fiber length and, accordingly, suffers from an inability to either zero-null the system or to increase the sensitivity of the system for a selected portion of the sensing fiber.